1. Field of the Invention
The present invention relates generally to ohmic contacts and more particularly to ohmic contacts for Al.sub.x In.sub.1-x As/Ga.sub.y In.sub.1-y As power HFETs.
2. Description of the Related Art
In typical heterostructure field-effect transistors (HFET), a layer of a high-bandgap semiconductor is epitaxially grown over a low-bandgap semiconductor layer. The upper layer is selectively doped to facilitate the formation of a potential well at the heterojunction interface. Electrons from ionized donors in the high-bandgap material are trapped in this well; they are free to move in a plane that is parallel with the heterojunction but are confined in a direction orthogonal to that plane. As a result, the electron system at the heterojunction has a two-dimensional characteristic which is typically referred to as a two-dimensional electron gas (2DEG).
This sheet of conduction electrons forms a channel between the source and drain of the HFET and electron flow through this channel is modulated by the field effect from a gate electrode. Because the conduction electrons are separated from the dopants of their high-bandgap material, they can move with essentially no ionized impurity scattering. Therefore, the electron mobility between the source and drain of the HFET is significantly enhanced, especially at lower temperatures where the ionized impurity scattering is the dominant scattering mechanism.
Two semiconductor systems that have demonstrated particularly promising HFET performance are the AlGaAs/GaAs and AlInAs/GaInAs systems (the semiconductors of the system are typically listed in a high-bandgap/low-bandgap order). In general, these two systems are fabricated respectively on GaAs and InP substrates. The former system is inherently lattice-matched to its substrate; the latter system is typically fabricated with an alloy composition of Al0.48In0.52As/Ga0.47In0.53As to achieve a lattice match with its InP substrate. Because of their high output voltage swings and high voltage bias points, power HFETs require a large gate-drain breakdown voltage. Higher breakdown voltages can be obtained with higher bandgap materials. Accordingly, power HFETs in the AlInAs/GaInAs system are generally fabricated with an alloy composition of Al.sub.x In.sub.1-x As/Ga.sub.y In.sub.1-y As with x&gt;0.48. The additional Al content increases the material's bandgap which produces the desired increase in breakdown voltage.
In any HFET semiconductor system, low-resistance source and drain ohmic contacts must be fabricated to communicate with the 2DEG channel in order to take advantage of the high-mobility electrons of that structure. N-type, ohmic contacts in the AlGaAs/GaAs system have typically been fabricated with a AuGeNi metal system which is alloyed to penetrate the high-bandgap semiconductor and reach the 2DEG.
In this semiconductor system, as reported by R. E. Williams (Williams, R. E., et al. "Ohmic Contacts", Gallium Arsenide Processing Techniques, Artech House, 1984, Chapter 11), it is theorized that Ga diffuses into the Au, which frees Ga crystal sites for occupation by Ge to form a narrow potential barrier which facilitates an ohmic current by tunneling emission. The Ni apparently serves as a wetting agent to prevent "balling up" of Au/Ge during alloying. The Ge is generally applied as a precompounded alloy of Au and Ge in a eutectic proportion. An overlay of Au is often used to improve the surface morphology of the contact.
A third HFET semiconductor system is the pseudomorphic AlGaAs/GaInAs system. A pseudomorphic structure has an intentional lattice mismatch which is accommodated by coherent layer strain when the layers are thinner than a critical thickness. C. S. Wu, et al. (C. S. Wu, et al. "Optimization of Ohmic Contacts for Reliable Heterostructure GaAs Materials", Journal of Electronic Materials, Vol. 19, No. 11, pp. 1265-1271) addressed ohmic contacts in the AlGaAs/GaAs semiconductor system and the pseudomorphic AlGaAs/GaInAs semiconductor system. They observed that the use of high temperature alloying to obtain penetration to the 2DEG resulted in high contact resistance and rough surface morphology in AuGe based metal systems. They tested the addition of a Ag diffusion barrier in a metal system which was deposited in the sequence of Ni (80 .ANG.)/AuGe(900 .ANG.)/Ag(1200 .ANG.)/Au(1200 .ANG.). This system produced a smooth surface morphology and a low contact resistance with 460.degree. C. alloying.
However, as reported by Capani, et al., (Capani, P. M., et al "Low Resistance Alloyed Ohmic Contacts", Electronics Letters, May 24, 1984, Vol. 20, No. 11) it has proven to be difficult to fabricate high-quality ohmic contacts in the AlInAs/GaInAs semiconductor system. In a first experiment with this semiconductor system, Capani, et al. sequentially evaporated a AuGeNi(1500 .ANG.)/Ag(1000 .ANG.)/Au(1500 .ANG.) metal system followed by a standard alloying technique. This produced an unacceptably high ohmic contact resistance (.about.10.sup.-4 .OMEGA.cm.sup.2). Subsequent analysis by Capani, et al. found gross Ag indiffusion into the semiconductor and significant In depletion. Capani, et al. theorized that the altered semiconductor stoichiometry resulted from the well known Ag-In affinity.
In a second experiment, Capani, et al. sequentially evaporated Ni (100 .ANG.)/Ge(365 .ANG.)/Au(800 .ANG./Ag(200)/Au(800 .ANG.), i.e., the Ag was reduced to avoid gross Ag-In interaction. Capani, et al. stated, "Some Ag was added as a solute for Au-Ag alloy formation which is known to reduce Au-Ga affinity as substantial Ga loss under ohmic contacts on GaAs is known to degrade contact resistance." Low contact resistance was obtained in this second attempt, which led Capani, et al. to conclude that the low resistance was obtained through, "--controlled alloying of AuGeNiAg metallisation. The metallisation was chosen so as to reduce Ga loss from the semiconductor by incorporating Ag within Au, the quantity of which was limited in order to prevent substantial In outdiffusion owing to In-Ag affinity."
In further comment on the use of Ag, P. Zwicknagl, et al. (Zwicknagl, et al. "Very Low Resistance Ohmic Contact Fabrication", Paper presented at 1th International Symposium on GaAs & Related CPDS, Biarritz, Sep. 25-28, 1994) also warned, "The presence of In in the AlGaAs/GaInAs system warrants considerable reduction in Ag in the ohmic metallization, the presence of Ag for the AlGaAs/GaAs case being required for the sake of reducing excessive Ga outdiffusion at higher temperatures."
The experiments of Capani, et al. emphasize that ohmic contact development must be approached independently for each semiconductor system. As stated by R. E. Williams (Williams, R. E., et al. loc. cit., pp. 232), "A complete understanding of ohmic behavior in metal-semiconductor contacts is yet to be achieved. Even though the underlying physics seem well established, exactly what occurs during actual processes used to fabricate ohmic contacts is much less clear. The importance of ohmic contacts for fabricating devices has inspired a great many experimental and empirical approaches to the problem."